Cross Platform WinRT+WP8

I am dealing with a project which it has two side,both WinRT and WP8.
My goal is taking ViewModels and Models to in a common area
(for now I'm using portable library for it)
for using same ViewModels and Models in both solution.

But When i am building the solution, i get some error like
"Cannot resolve Assembly or Windows Metadata file 'GalaSoft.MvvmLight.dll'"